The present invention relates to a four-wheel drive vehicle which is selectively driven in two-wheel drive mode or four-wheel drive mode.
A four-wheel drive vehicle of the type in which a transfer clutch is provided for transmitting the output torque of a transmission to auxiliary drive wheels is known. In such a vehicle, a power transmission system can be converted from a two-wheel drive system to a four-wheel drive system by engaging the transfer clutch. However, when the vehicle negotiates corners by the four-wheel drive system, braking phenomenon called "tight corner braking" will occur at a sharp corner. This is caused by greater radius of front wheels than that of rear wheels and therefore tendency to rotate faster than the rear wheels.
Recently, it is proposed to use a multiple-disk type hydraulic coupling filled with a viscous material (hereinafter called viscous coupling) as a transfer clutch. The viscous coupling has a characteristic that transmitting torque increases with increase of the difference between input speed and output speed. Accordingly, the viscous coupling acts to prevent the tight corner braking because of small difference between input and output speeds and to prevent slipping of wheels on slipperly roads because of large difference between input and output speeds.
Japanese Utility Model Application Laid Open No. 59-188731 discloses a four-wheel drive vehicle in which the output of a front-mounted engine is transmitted to rear wheels through a viscous coupling. Accordingly, the rear wheels are driven only when the speed of front wheels (V.sub.F) is larger than the speed of rear wheels (V.sub.R). In other words, in ordinary driven conditions, the vehicle is driven by front wheels only. Although the front-wheel driving is superior in steerability, driveability of the vehicle, particularly at starting of the vehicle at heavy load, is inferior compared with the rear-wheel driving.